About 76% of Americans aged 65 and older have received at least one shot of the COVID-19 vaccines since authorization in December, according to the Centers for Disease Control and Prevention. But the rate of new vaccinations among the group most vulnerable to adverse virus outcomes has dramatically slowed. It’s a growing source of concern, not only because of the potential for preventable deaths and serious illness among seniors in coming months but also for what it could portend for America’s broader population. “I want to make a direct appeal to our seniors and everyone who cares about them,” President Joe Biden said Tuesday, citing “incredible progress” but declaring it’s still not enough. “It’s simple: Seniors, it’s time for you to get vaccinated now. Get vaccinated now.”